Maximization of hydrogen peroxide formation during water electrolysis

Due to its high availability, water is considered the most useful starting material for hydrogen production. Ideally, the conversion of water into hydrogen produces a second useful substance: hydrogen peroxide, which is required in many industries, such as the production of disinfectants.

Special reaction conditions are required to obtain hydrogen peroxide from water splitting. The presence of carbonate was known to be beneficial. But why this is so was not clear. A team from Ruhr University Bochum in Germany has now elucidated the mechanism behind this.

The group led by Dr. Lejing Li, Dr. Carla Santana Santos and Professor Wolfgang Schuhmann from the Center for Electrochemistry at Ruhr University Bochum describe the results in the journal Angewandte Chemie International Edition.

Kill two birds with one stone

“Hydrogen peroxide is a valuable chemical that must be produced using complex processes that are not always environmentally friendly,” says Schuhmann.

It would be useful if the substance could be obtained in large quantities from the electrolytic splitting of water, which also produces the energy carrier hydrogen. “However, this is thermodynamically complicated,” explains Lejing Li. The production of oxygen is, so to speak, easier energetically.

However, if a carbonate buffer is added to the solution, the situation changes. It is carbonic acid (H₂WHAT₃), which can release a proton (H⁺), leading to bicarbonate (HCO₃^–), which can further react to form carbon dioxide (CO₂). Such buffers help keep the pH value of the solutions stable. However, the conditions in the reaction solution are not the same everywhere.

The conversion of water into hydrogen and oxygen takes place on the surfaces of two electrodes, between which a voltage is applied. During the transfer of negatively charged electrons, positively charged protons are simultaneously released. Protons change the pH value in the immediate vicinity of the electrode, while further in the solution the pH value remains stable.

Local measurement of the pH value

Using a method they developed themselves, the team from Bochum determined the pH value in the immediate vicinity of the electrode under different reaction conditions and showed that hydrogen peroxide is preferentially produced when there is a lot of bicarbonate in the vicinity of the electrode. Under these conditions, an intermediate reaction product is formed, which prevents the formation of unwanted oxygen.

“At first, these results sound like abstract basic research,” says Li. “But the production of hydrogen and hydrogen peroxide is extremely important. Only if we understand the processes exactly can we improve them.”